1. Field of the Invention
This invention relates to digital photography, and more particularly to a low power analog-to-digital converter for a multiple-sampling digital pixel sensor.
2. Description of Related Art
Digital photography is one of the most exciting technologies to have emerged during the twentieth century. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make the digital cameras one of the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the same way as traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an array of pixel sensors or photosensitive devices, such as a charged-coupled devices (CCD) or complementary metal-oxide semiconductor (CMOS) sensors to sense an image. The array of these pixel sensors are arranged in the focal plane of the camera such that each sensor produces an electrical signal proportional to the light intensity received at its location.
The image thus produced has a resolution determined by the number of sensors in the array. A modern digital camera may have a million or more of these sensors. The resulting digital image has discrete picture elements (pixels) corresponding to the number of sensors in the array. Because of the correlation, the sensor elements themselves are often referred to as pixel sensors as well. The light intensity received by each pixel sensor is expressed as a discrete value.
It is known to organize the pixel sensor array in two dimensions, addressable by row and column. Once a row of elements has been addressed, the analog signals from each of the pixel sensors in the row are coupled to the respective columns in the array. An analog-to-digital converter (ADC) may then be used to convert the analog signals on the columns to digital signals so as to provide only digital signals at the output of the array, which is typically formed on an integrated circuit.
Because of a number of problems such as degradation of signal and slow read out times in prior art pixel sensor arrays, a xe2x80x9cdigital sensor pixelxe2x80x9d has been developed as described in, e.g., U.S. Pat. No. 5,461,425, which is hereby incorporated by reference. FIG. 1 illustrates an array 12 of digital sensor pixels 14 on an integrated circuit 10. Each digital sensor pixel 14 in the array 12 includes a photodetector and a dedicated ADC such that the pixel 14 outputs a digital rather than an analog signal as in prior art pixel sensor arrays. In contrast, prior art pixel sensor arrays did not have a dedicated ADC for each individual sensor in the array. Digital filters 16 on integrated circuit 10 are connected to receive the digital output streams from each digital pixel sensor 14 and convert each stream to, e.g., an eight-bit number representative of one of 256 levels of light intensity detected by the respective digital pixel sensor 14. Within the digital pixel sensor 14, the analog signal from the phototransistor is converted into a serial bit stream from its dedicated ADC clocked using a common clock driver 18. The digital filters 16 process the bit stream from each digital pixel sensor 14 to generate an eight-bit value per pixel element 14. These eight-bit values may then be output from the chip 10, using a suitable multiplexer or shift register, and temporarily stored in a bit-mapped memory 24.
Because a digital signal is produced directly by the pixel 14, several advantages over the prior art become apparent. For example, dynamic range is a critical figure of merit for pixel sensors used in digital cameras. The dynamic range of an pixel sensor is often not wide enough to capture scenes with both highlights and dark shadows. This is especially the case for CMOS sensors that, in general, have lower dynamic range than CCDs.
To address the need for increased dynamic range, U.S. Ser. Nos. 09/567,786 and 09/567,638, both filed May 9, 2000 and incorporated by reference herein, disclose an architecture for the digital pixel sensor in which the dynamic range of the sensor is increased by taking multiple samples of a subject during a single imaging frame, where each sample is taken over an interval of a predetermined duration. As used herein, a xe2x80x9cframexe2x80x9d shall denote the period of time during which the multiple exposures for a single image occur as formed by an array of digital pixel sensors.
Although this multiple sampling method provides an increased dynamic range, it may suffer from excessive power consumption by its analog-to-digital converters. For example, if a pixel""s image sample within a given frame approaches or reaches saturation, subsequent image samples from this same pixel within this frame provide no further information because they too will be saturated. Despite their saturation, however, the analog-to-digital converters will continue to process these signals, needlessly wasting power.
Accordingly, there is a need in the art for a digital imaging system whose analog-to-digital converters do not process signals from pixels that are saturated.
In accordance with one aspect of the invention, a system for performing low-power analog-to-digital conversion in a time-indexed multiple sampling digital imaging system is presented using an array of pixel sensors. Multiple exposure times are used for each pixel sensor to form an image during a single frame. An analog image signal is produced by each pixel sensor for each exposure time wherein, prior to satisfaction of a threshold value, an analog-to-digital converter converts each analog image signal into a corresponding digital image signal. The analog image signals are tested against a threshold value. If the threshold value is exceeded for a given pixel sensor, its analog-to-digital converter is prevented from converting the pixel sensor""s analog image signals from subsequent exposure times in the frame. However, those pixel sensors whose analog image signals have not exceeded the threshold value will continue to be converted by their analog-to-digital converters for this frame.
In accordance with another aspect of the invention a method of low power analog-to-digital conversion using a time-indexed multiple sampling technique is presented. The method uses a time-indexed multiple-sampling digital imaging system having an array of pixel sensors. Each pixel sensor in the array produces a plurality of analog image signals corresponding to a plurality of exposure times used for each frame. In addition, an analog-to-digital converter is configured to convert the analog image signals into corresponding digital image signals. The method includes a step of testing, after each exposure time, whether the corresponding analog image signal from each pixel sensor satisfies a threshold value. If the threshold is satisfied for a given exposure time in a given pixel sensor within the array, the method includes a further step of preventing the analog-to-digital converter from converting analog image signals from this pixel sensor resulting from exposure times subsequent to the given exposure time in the frame.
The invention will be more fully understood upon consideration of the detailed description below, taken together with the accompanying drawings.